1. Field of the Invention
The present invention relates to a sample diluent for use in determination of ion concentration using ion-selective electrodes, and to a method of using the same. In particular, it relates to a diluent suitable for use in determining ion concentration in body fluids such as blood, urine, etc. with ion-selective electrodes, and to a method of determining ion concentration which uses the diluent.
2. Description of the Related Art
In the analysis of body fluids such as blood, urine, etc. in laboratories of hospitals or such, a vast number of analyses are made for sodium cation, potassium cation and chlorine anion. For the determination of these ions, flame analysis and coulometric titration long have been used. However, in recent years, the ion-selective electrode method has come to be in wide use. In using ion-selective electrodes, the sample is usually diluted to an appropriately low concentration with a diluent prior to the determination. The dilution is usually necessary for decreasing the amount of the sample such as blood or for adjusting the concentration of the constituents to be subjected to the measurement to the range within which the electrodes fully exhibit their capacity. The factor of dilution is generally from ten-fold to several ten-fold, although it may vary depending on the kind of autoanalyzer to be used, or for other reasons.
Due consideration has not been given to the hydrogen ion concentration (namely, pH) of the samples diluted with the conventional, commercially available diluents. Therefore, the pH of the diluted sample to be analyzed often varies from sample to sample. Particularly, when urine which originally has a wide variation of pH among different samples is used as a sample, the pH of the diluted sample to be analyzed varies widely, leading to a large error in the results of ion concentration determination. Accordingly, in order to enhance the accuracy of ion concentration determination, the improvement of diluents in this respect has been eagerly desired.
JP-A-60-228951 and JP-A-61-290353 disclose the use of a mixture of tris(hydroxymethyl)aminomethane and boric acid as a sample diluent in determination of ion concentration using ion-selective electrodes.